A camera having a wide dynamic range, i.e., a so-called wide dynamic range camera capable of clearly sensing motion images of both a very bright object and dark object in the same angle of view is attracting attention.
On the other hand, the field of still images also has a technique which corrects an object having a brightness difference, particularly, a backlighted object by digital image processing corresponding to a dodging process performed in a darkroom for silver-salt films. For example, Jobson et al., “A Multiscale Retinex for Bridging the Gap Between Color Images and the Human Observation of Scenes” (IEEE TRANSACTIONS on IMAGE PROCESSING, VOL. 6, NO. 7, JULY 1997) is a method which improves a digital image by performing differential processing between a logarithmically converted component of the image and a low-frequency component of this logarithmically converted component, thereby darkening a bright component in a low-frequency region of the image, and brightening a dark component in the low-frequency region.
Also, Richard et al., “Photographic Tone Reproduction for Digital Images” (acm Transactions on Graphics, JULY 2002, Vol. 21, No. 3) proposes a method which obtains an effect similar to dodging in digital image processing by using a luminance component and low-frequency component of a digital image.
Furthermore, a so-called network camera by which a camera is connected to a network and an image in a remote place is viewed by using a dedicated or general-purpose viewer is recently attracting attention (Japanese Patent Laid-Open No. 10-040185). When the dodging process is applied to this network camera, the same effect as the wide dynamic range camera can be given to the network camera.
Since, however, the dodging process for still images is a process which brightens a dark portion, noise components contained in an image obtained by an image sensor using a charged-coupled device (CCD) or the like sometimes become conspicuous. The larger the amount of correction which brightens a dark portion, the more conspicuous the noise components buried in the dark portion of the image. Accordingly, if the dodging process is applied to motion images sensed by a motion image camera such as a monitor camera or network camera, temporal fluctuations are further added to noise which is already made conspicuous by dodging, thereby producing noise which flickers frame by frame. As a consequence, the noise is observed as very conspicuous random noise.
If the performance of the image sensor improves in the future to eliminate most noise, the problem as described above may be solved. At present, however, the dodging process often makes the existing buried latent noise conspicuous.